JavaMail: javax.mail.AuthenticationFailedException

I'm having troubles getting JavaMail to work with authentication, using an outside SMTP-server. The server is not the server of my ISP (the one which connects me to the internet now). I can use this SMTP-server to send mails with Thunderbird, so the connection should work.

I get the following error:

javax.mail.AuthenticationFailedException
      at javax.mail.Service.connect(Service.java:306)
      at javax.mail.Service.connect(Service.java:156)
      at javax.mail.Service.connect(Service.java:105)
      at javax.mail.Transport.send0(Transport.java:168)
      at javax.mail.Transport.send(Transport.java:98)
      at net.grexx.square.mail.SimpleSender.send(SimpleSender.java:66)
      at net.grexx.square.mail.ProcessSqlMail.process(ProcessSqlMail.java:67)
      at org.apache.jsp.SimpleSender.test_002ddb_jsp._jspService(org.apache.jsp.SimpleSender.test_002ddb_jsp:46)
      at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:99)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
      at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:325)
      at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:295)
      at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:245)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:237)
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:157)
      at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:214)
      at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
      at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
      at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
      at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
      at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
      at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:825)
      at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:731)
      at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:526)
      at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
      at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
      at java.lang.Thread.run(Unknown Source)

The java code that sends the mail is:

      public static void send(String smtpServer, String smtpUsername, String smtpPassword, String to, String from
                  , String subject, String body)
      {
            try
            {
                  Properties props = System.getProperties();
                  // -- Attaching to default Session, or we could start a new one --
                  props.put("mail.smtp.host", smtpServer);
                  props.put("mail.smtp.auth", "true");
                  Session session = Session.getDefaultInstance(props, null);
                  // -- Create a new message --
                  Message msg = new MimeMessage(session);
                  // -- Set the FROM and TO fields --
                  msg.setFrom(new InternetAddress(from));
                  msg.setRecipients(Message.RecipientType.TO,
                  InternetAddress.parse(to, false));
                  // -- We could include CC recipients too --
                  // if (cc != null)
                  // msg.setRecipients(Message.RecipientType.CC
                  // ,InternetAddress.parse(cc, false));
                  // -- Set the subject and body text --
                  msg.setSubject(subject);
                  msg.setText(body);
                  // -- Set some other header information --
                  msg.setHeader("X-Mailer", "GrexxBoxxEmail");
                  msg.setSentDate(new Date());
                  
                  Transport tr = session.getTransport("smtp");
                  tr.connect(smtpServer, smtpUsername, smtpPassword);
                  msg.saveChanges();      // don't forget this
                  
                  // -- Send the message --
                  Transport.send(msg);
                  System.out.println("Message sent OK.");
            }
            catch (Exception ex)
            {
                  ex.printStackTrace();
            }
      }

I have no idea what I'm doing wrong here. Any suggestions?